1. Field of the Invention
This invention relates to a residual roll film detection device for detecting the amount of usable roll film remaining on a supply reel in a camera.
2. Description of the Related Art
The microfilm camera is used for photographing original images recorded in documents of various forms on a microfilm. The microfilm camera basically comprises a document table supporting the documents mentioned above and a camera head disposed above the document table.
The interior of this camera head is so adapted that a supply reel having wound thereon the unused part of a roll of microfilm or roll film and a take up reel intended to take up thereon the part of the roll film on which original images recorded on documents have been already photographed will be detachably mounted therein. As the work of photographing original images on the microfilm by the use of the microfilm camera constructed as described above proceeds, the roll film moves from the supply reel to the takeup reel. When the part of the roll film wound on the supply reel eventually ceases to exist, the roll film thus used up must be replaced with a new roll film.
In order for the work of preparing the new roll film for insertion in the camera to be carried out with expedition, the microfilm camera is furnished with a device capable of externally detecting the length of the part of the roll film still wound on the supply reel, namely, the amount of the residual roll film.
The outside diameter of the part of the roll film still wound on the supply reel is directly in proportion to the remaining length of the roll film. It has been, therefore, customary to indicate on a display part the remaining length of the roll film calculating from the outside diameter of the roll film. The idea of detecting the residual amount of roll film based on the diameter as mentioned above is basically equal to the technique employed in detecting the residual amount of roll paper in a copying machine using a roll paper as disclosed in Japanese Patent Laid-Open Application SHO No. 58(1983)-75,164 or in detecting the residual amount of roll paper in a laser printer using a roll paper as disclosed in Japanese Patent Laid-Open Application SHO No. 59(1984)-68,748. In the conventional copying machine mentioned above, a roll paper holder is provided on one lateral face thereof with a partially cutaway display part and the display part is furnished in the proximity thereof with an indicating scale. The operator is allowed to find the residual length of roll paper by visually reading the outside diameter of the roll paper against this scale. The printer mentioned above is provided with two residual roll paper detecting pieces and is enabled to indicate the residual amount of roll paper by the fact that one of the detecting pieces left in contact with the peripheral surface of the roll paper eventually comes into contact with the other detecting piece and established electric conductivity when the diameter of the roll paper decreases to a prescribed value.
The roll film heretofore used in the microfilm camera has had a limited thickness, t, of 5 mils (5/1000 inch or 0.127 mm). For the purpose of increasing the capacity of one roll film for original images to be photographed thereon, a roll film having a smaller thickness of 2.5 mils has been developed. It has aroused an earnest desire to provide a camera head which is usable convertibly with the roll film of this small thickness and the conventional roll film of large thickness.
In the conventional system in which the diameter of a roll film wound on a reel is directly converted into the residual amount of this roll film and the result of this conversion is indicated on the display part, however, since the residual amount calculated from the diameter is variable with the thickness of film, the display of the exact residual amount is not obtained with respect to either of two roll films differing in thickness.